Among the many problems confronted upon application of the Stirling engine to motor vehicle useage, two are of particular concern because they effect overall vehicle design. Problem (1) concerns the kind and type of equipment to be employed for alleviating the engine of its excess heat; and problem (2) is what equipment shall be used to heat the passenger compartment. In a Stirling engine, the engine cooling water must be regulated to a lower temperature than that in today's internal combustion engine; that is, the lowest temperature to which the cooling water is cooled by radiation is designed to be much lower. This will adversely affect the passenger heating system which depends on engine waste heat; the radiator or heater core employed for the passenger compartment will be too small to facilitate proper heating of the vehicle compartment in winter. In addition, the Stirling engine rejects more heat to the atmosphere and, this fact, coupled with the lower cooling water temperature, causes the demand upon the radiator and fan to become more severe especially in the summer where a greater volume of heat must be rejected to atmosphere.
Tomorrow's automobile must make the most efficient use of every pound it transports. Thus any solution to the above vehicle design problems must properly accommodate the heating of the passenger compartment and the cooling of the Stirling engine, and must take into consideration that weight must be incorporated only at a very minimum level. This is a direct result of diminishing fuel supplies and the emission sensitivity of automobile design. Unnecessary weight can be ill-afforded from either the performance or economy point of view.
The typical automobile, produced today, has a passenger compartment heater core in the climate control system which is utilized only about 50% of the time that the vehicle is driven, assuming a typical northern climate. If the radiator water is to be maintained cooler in a Stirling engine application, then the heater core will have to be increased in size (with correspondingly added weight) to achieve a proper heat transfer for the vehicle passenger compartment. Even if the initial cost of this additional apparatus is neglected, the added weight is unfortunate when viewed from the standpoint that it is 50% utilized. There is a similar or companion increase in size and weight required for the radiator to meet the peak heat rejection conditions for the Stirling engine prevailing in summer in a typical northern climate. Again this added weight can be considered a penalty since the additional weight is utilized only about 50% of the time of the functioning unit. If the heater core increase in size can be employed in the winter for heating the passengers and again in summer to augment the radiator during peak loads, then the radiator may be maintained at a minimum weight and size.